The present invention relates generally to mobile message receivers, commonly referred to as pagers, of the type which are battery-powered and which monitor pager transmissions encoded with address and message information directed to one or more of the pager units, and more particularly to a system and method for analyzing the coded transmissions sent to pagers to enable the pagers to retrieve any message with a minimum expenditure of battery power.
Mobile message receivers, referred to herein as pagers, are personal message receiving devices which monitor certain broadcast channels used by paging networks to alert or send information to specific pager units. Pagers periodically monitor the channel or channels used by the network to which it is assigned to search for messages. When an incoming message is identified, the pager alerts the user, or displays the message, or otherwise responds in accordance with the message information and the pager's capabilities. Pager units are small, lightweight, battery-operated devices which incorporate power-consuming elements. A major user of battery power is the RF receiving circuit. To reduce the size of the pager batteries, or to extend their useful life, various pager address and message coding techniques have been developed to minimize the time when the power-draining radio receiver is turned on. One widely-used digital broadcast standard, designed to facilitate battery savings and provide for reliable identification of incoming messages, is known by the acronym POCSAG. Developed by British Telecom in the United Kingdom, POCSAG (for "Post Office Code Standardization Advisory Group") is a broadcast standard for signaling pagers and other mobile message receivers. The POCSAG encoding standard uses a transmission format which begins with a preamble code of a predetermined duration (the standard specifies 576-bits) followed by one or more batches of codes, each batch containing a synchronization code followed by 16 code words in 8 predetermined timeslots, or frames. All pagers in a paging network are assigned to one of eight groups corresponding to one of the timeslots in a batch. A pager's address code will only be transmitted in its designated timeslot, which allows the radio circuit in the pager to be turned off most of the time, thus extending the life of the pager's batteries.
Because the POCSAG standard is used in many pager systems, numerous decoding and battery saving protocols have been developed for POCSAG. In general, all prior art battery saving techniques seek to reduce to a minimum the time when the radio receiving circuit is energized, while avoiding inadvertent loss of any messages directed to the pager. It would be advantageous if the POCSAG coded transmissions could be efficiently analyzed in an improved way to further reduce power consumption.
It would also be advantageous to provide a POCSAG decoding system for pagers in which, when the pager is operating in a synchronized mode and its internal clock is synchronized with the pager transmission, the decoder is able to make a bit-by-bit analysis of each code word in the pager's designated timeslot to determine if any of the pager's addresses are being sent. That would allow the pager's radio circuit to be turned off before an entire code word is received.
It would also be advantageous to provide a pager decoding methodology which can determine whether the pager's internal clock remains synchronized with the coded transmission without having to energize the pager's radio receiver to receive the synchronization codes embedded at regular intervals in POCSAG transmissions.
Accordingly, the present invention provides a decoder for use in a mobile message receiver, such as a pager, which receives and processes coded transmissions of a type that selectively include a preamble code and batches of other codes, each such batch including a synchronization code followed by a plurality of code words in predetermined timeslots. One or more of the timeslots are the designated timeslots where one or more of the receiver's addresses is transmitted when information is sent. The receiver also includes an internal clock which is synchronized with a coded transmission when a synchronization code is received. The clock, when synchronized, allows the pager to identify selected bit positions within a transmission. A radio circuit is provided which is selectively enabled to receive a coded transmission and produce a coded signal therefrom. Such coded transmissions are transmitted at various selected baud rates, one or more of the baud rates being the correct baud rate for the pager decoder. The decoder comprises a baud rate detector for monitoring the coded signal from the radio circuit to determine if the baud rate of the coded transmission corresponds to a correct baud rate for the decoder. A preamble detector is provided for monitoring the coded signal to detect preamble code. An address detector is provided for monitoring the coded signal to determine if there is at least a predetermined correlation between the coded signal and the receiver's address. And an enabling controller is provided which is responsive to the receiver's radio circuit, the baud rate detector, the preamble detector, and the address detector for selectively enabling and disabling the radio circuit when the receiver is in a synchronized mode of operation wherein the internal clock of the receiver is synchronized with a coded transmission. The enabling controller enables the radio circuit at the start of each code word in the designated timeslots of the receiver, and disables the radio circuit before an entire code word has been received, if: (a) the baud rate of the batch is determined to be correct; and (b) the coded signal is not a preamble code; and (c) there is less than a predetermined correlation between the coded signal being received and the receiver's address.
It is a feature of the present invention that the receiver frequently samples the transmissions being received for baud rate information and for the presence or absence of preamble code. Whenever the receiver is receiving and the decoder is decoding code words in the receiver's designated timeslots, the present invention uses baud rate and preamble code information to confirm that the receiver's internal clock remains synchronized with the transmission. Continued synchronization can be reliably inferred from a correct baud rate, and the absence of preamble code in the signal being received. The invention allows the pager to reliably receive and analyze only a part of a code word in the pager's designated timeslot before turning off the radio receiving circuit, if the pager and transmission are synchronized. Repeated checks of synchronization are necessary to establish that the pager is reading the start of actual addresses. The invention monitors synchronization without the need to repeatedly enable the radio circuit each time a synchronization code is transmitted. Instead, it derives synchronization information by sampling the baud rate and determining the presence or absence of preamble code, which can be done when the radio circuit is enabled to receive a pager transmission.
In its preferred form, the enabling controller of the pager begins operating in what is called the search mode, or the signal validation mode. In that mode the pager radio circuit is enabled to search for coded transmissions which have the correct baud rate for the decoder. The radio circuit is enabled periodically and responds to the baud rate detector and the preamble detector each time the radio circuit is enabled, to determine if the coded transmission being received is compatible with the decoder. Based on the outputs of the baud rate detector and preamble detector, the controller (1) continues to enable the radio circuit until the receiver's clock becomes synchronized with the coded transmission if either the baud rate of the transmission is correct, or a preamble code is detected; or (2) if the baud rate is incorrect and preamble codes is not detected, it disables the radio circuit. More particularly, the enabling controller responds to the baud rate detector and the preamble detector, as follows: (1) if either the baud rate is correct or preamble code is detected, the enabling controller continues to enable the radio circuit until the receiver's clock becomes synchronized with the coded signal; or (2) if the baud rate is incorrect and preamble code is not detected, the enabling controller disables the radio circuit after an interval sufficient to verify the aforementioned baud rate and preamble code information.
The other preferred mode of operation is called the synchronized mode. When the pager is operating in the synchronized mode the pager's clock is synchronized with the pager transmission and the radio circuit is enabled to receive the start of each code word in the pager's designated timeslot or timeslots and is disabled before an entire code word has been received, if: (a) the baud rate of the batch is determined to be correct; and (b) the coded signal is not a preamble code; and (c) there is less than a predetermined correlation between the coded signal being received and the receiver's address. The controller continues to enable the radio circuit long enough to receive substantially an entire code word if either of the following occurs: (1) if the baud rate is determined to be incorrect; or (2) if preamble code is detected. If either the baud rate is incorrect or preamble code is detected, it is likely that the clock has become unsynchronized with the coded transmission. If the baud rate is incorrect after reading the entire code word, a mathematical analysis of the code word is performed to determine if it is a valid code word. A valid code word will not appear unless the receiver's clock is still synchronized. Consequently, the receipt of a valid code word means the pager's clock is still synchronized. A code word validator is provided to determine if the transmission being received contains a valid code word. Whenever the validator determines that the receiver is receiving valid code words, the enabling controller continues to operate in the synchronized mode and enables the radio circuit at the start of each code word in the pager's designated timeslots. Alternatively, when the code word validator determines that a predetermined number of consecutive code words being received are not valid code words, the enabling controller concludes that synchronization has been lost and discontinues operating in the synchronized mode and returns operation to the signal validation mode.
The invention also includes a method for minimizing the energy consumed by a mobile message receiver, or pager, having a radio circuit and an internal clock and which receives and decodes digitally coded transmissions of the type which selectively include a preamble code followed by batches of other codes. The transmission batches each include a synchronization code followed by a plurality of code words in selected timeslots, at least one of the timeslots being a designated timeslot in which an individual pager's one or more addresses are transmitted when information is sent. Such coded transmissions are sent to such receivers at various selected baud rates, one or more of which is a correct baud rate for the pager decoder. A first embodiment of the method is carried out when the receiver's clock is synchronized with a selected coded transmission, and comprises a first step of enabling the radio circuit at the start of each code word in the designated timeslot or timeslots of the receiver. Another step is to periodically determine if the baud rate of the coded transmission corresponds to a correct baud rate for the receiver. This last-mentioned step is preferably performed once in each batch. Another step is to determine if the coded transmission is preamble code. The two last-named steps, i.e., determining if the baud rate is correct and determining if the transmission is a preamble code, together provide information about whether or not the receiver's internal clock remains synchronized with the coded transmission. Another step is to determine the amount of correlation between the incoming bits of the coded transmission and the receiver's addresses. The three last-named steps are preferably performed substantially simultaneously. The purpose of the step of determining the correlation between incoming bits and the pager's addresses is to determine if the difference between the bits being received and the pager's addresses is sufficiently great to preclude the presence of the pager's address code. The radio circuit can be turned off even before an entire code word has been received if the transmission is not an address code. Consequently another step in the method is to disable, i.e., turn off, the radio circuit before an entire code word has been received if: (a) the baud rate of the batch is determined to be correct; and (b) the coded transmission is not a preamble code; and (c) there is less than a predetermined correlation between the transmission and the pager's address. The information in (a) and (b) determines whether the pager's clock is still synchronized with the transmission. Assuming that the receiver's address is a predetermined bit pattern, the aforementioned step (c) in the step of disabling the radio circuit before an entire code word has been received, preferably further includes the following steps: (1) identifying each bit of the coded transmission as it is received; (2) comparing each bit against the bit patterns of the pager's addresses; and (3) determining if the pattern of the received bits is sufficiently different from the patterns of bits in the pager's addresses to insure that an address is not being received. If the incoming bits do not match a stored pager address, and if the clock remains synchronized, the radio circuit is turned off before the entire code word is received.
The method also provides a signal validation mode in which the pager searches for compatible transmissions. Steps in the method include periodically enabling the radio circuit and: (1) determining if the baud rate of the coded transmission is the correct baud rate for the receiver; and (2) determining if the coded transmission is a preamble code; and (3) using the baud rate and preamble code information to selectively enable the radio circuit to synchronize the receiver's clock with the coded transmission. Once the pager's clock is synchronized with the coded transmission, the pager then continues to operate in accordance with the synchronized mode in which the radio circuit is enabled to receive the code words in the pager's designated timeslots,